Conjugated dienes are of considerable importance in organic chemistry in and of themselves. In addition, they are extremely important for use in the well known Diels-Alder reaction. Many conjugated dienes, biaryls and polyenes are used as intermediates for synthesis reactions and as monomers for the formation of polymeric reaction products. For example, the preparation of polybutadiene rubber.
One problem often encountered with prior art processes for the formation of conjugated dienes is that the reaction procedures often are unsuitable for the preparation of functionally substituted dienes. Thus, if the diene being prepared is functionally substituted with, for example, a carboxyl group, a carbonyl group, an amino group, an ester group, or the like, often the reactive site in any synthesis reaction will be at the functional group rather than the formation of the desired conjugated diene. As a result, very few functional groups have been incorporated into these reactions.
In addition, the preparation of conjugated dienes often encounters the difficulty that the stereospecificity of the reaction starting material is lost in the coupling procedure to prepare the conjugated diene. This is important in many syntheses since the stereochemistry can and indeed often does affect the ultimate reaction properties of any polymers which are formed.
Accordingly, there is a real need in the art for the development of a new process for the preparation of symmetrical conjugated dienes and polyenes which both tolerates functionality and which produces symmetrical dienes, stereospecifically, in high yields. This invention has as one of its primary objectives the satisfaction of the above described needs, with respect to synthesis of conjugated dienes by a catalytic synthesis procedure.
My earlier application related to a useful new method for the symmetrical dimerization of readily available vinylmercuric chlorides which employed stoichiometric amounts of palladium chloride and lithium chloride and provided 1,3-dienes in excellent yield. It has now been found that similar results can be achieved by use of rhodium catalysts thereby eliminating the previously required stoichiometric amount of expensive palladium chloride.
It has also been discovered that nearly quantitative yields of biaryl compounds can be obtained by dimerization of aryl-mercurials in accord with a similar rhodium catalyzed reaction.